1. Field of the Invention
The present invention relates to a method and apparatus for making an optical fiber whose transmission loss is lowered by reducing its Rayleigh scattering intensity.
2. Related Background Art
As a method of making an optical fiber whose transmission loss is lowered by reducing its Rayleigh scattering loss, one disclosed in Japanese Patent Application Laid-Open No. HEI 10-25127 has been known, for example. In this manufacturing method, an optical fiber preform is drawn upon heating so as to prepare an intermediate optical fiber, which is then reheated so as to be subjected to a heat treatment, whereby the reheating lowers the virtual temperature (the temperature to which the randomness of the state of atomic arrangement within the glass corresponds) due to the structural relaxation (rearrangement of atoms) in the glass, thus reducing its Rayleigh scattering intensity.
However, the surface of the optical fiber immediately after drawing is coated with a UV resin or the like in order to protect the heated and drawn optical fiber. The above-mentioned method of making an optical fiber disclosed in Japanese Patent Application Laid-Open No. HEI 10-25127 is not suitable for mass-producing coated optical fibers, since the resin coated on the optical fiber surface is burned by the heat at the time of reheating. Though the optical fiber may be reheated while in an uncoated state, it is not applicable as a mass-producing method due to damages at the time of handling the optical fiber and the like.